The Great Unconformity

Frenchman Mountain Southern Nevada

http://www.geotimes.org/nov05/Travels1105.html

http://www.terraprints.com/catalog/nevada_satellite_map.jpg

Or, how I

learned

Why we need

Nevada!

• What is an

Unconformity?

• What is the great

Unconformity of

Frenchman mtn?

• What creates them?

• Where did the name

come from?

• An unconformity is a buried erosion surface. It is a surface of rock that was exposed on

the Earth's surface and was then covered by younger layers.

• Unconformities are important because they represent missing intervals of the geologic

record, like pages missing from a history book. The Great Unconformity of Frenchman Mountain represents about 1.2 billion years, which is more than one fourth of the age of

the Earth.

http://geoscience.unlv.edu/pub/rowland/Virtual/geology.html#whatis

• The concept of an unconformity arises from two of the oldest principles of geology, first stated in 1669 by Nicholas Steno:

• Layers of sedimentary rock (strata) are originally laid down flat, parallel to the Earth's surface. That's the law of original horizontality.

• Younger strata always overlie older strata, except where the rocks have been overturned. That's the law of superposition.

http://geology.about.com/od/geoprocesses/a/unconformities.htm

Frenchman Mountain

http://www.idahosummits.com/frenchman/images/frenchman%20mountain.jpg

LAS VEGAS “BOWL”

EAST- Frenchman

Mountain Range;

WEST - Spring

Mountain Range;

NORTH - Sheep

Mountain Range;

SOUTH - McCullough

Mountain Range

http://nevada-history.org/facts.html

http://geoscience.unlv.edu/pub/rowland/Virtual/hike.html

Forming the Great Unconformity

http://geoscience.unlv.edu/pub/rowland/Virtual/geology.html#name

FROM THE GRAND CANYON TO THE LAS VEGAS

BOWL…HOW A MOUNTAIN MOVES.

6-10 Million years ago western North America begins

moving west due to interaction between the North

American and Pacific lithospheric plates, which today

slide past one another along the San Andreas fault in

California.

Movement of blocks becomes tilted as secondary faults

encounter a nearly horizontal master fault causing the

blocks to tilt eastward while moving westward.

Continuous sliding on the master fault causes

Frenchman Mountain to move about 50 miles from

original position.

http://geoscience.unlv.edu/pub/rowland/Virtual/geology.html

Precambrian granite (about 1.7 billion years old) and beginning with the white rocks exposed along the slope of the low hill, are the Cambrian

Tapeats Sandstone (about 500 million years old).

• Above the Great Unconformity we

see the sedimentary Tapeats

Sandstone with recycled large, light

gray, angular, quartz pebbles.

These rocks have a dark coating

called desert varnish or rock varnish

which is a thin manganese- and iron-

rich coating that develops on exposed

rock surfaces and is most common in

desert regions.

Bacteria, using chemicals from dust

particles,[produces this varnish over

thousands of years

http://geoscience.unlv.edu/pub/rowland/Virtual/hike.html

FRENCHMAN MOUNTAIN WAS PARTLY CREATED BY

MOVEMENT ALONG THE FRONT RANGE FAULT

SHOWN BELOW

http://www.wheaton.edu/ACG/trip/jpgs/35b.jpg

http://img.geocaching.com/cache/log/423d19f3-5359-49b0-97ea-25560519bb36.jpg

Granite intrusion in the schist.

http://img.geocaching.com/cache/log/577d4037-28e0-4cc6-8aaa-9433f8aaf221.jpg

Precambrian granite and schist / Tapeats sandstone

Some old guy and The Great Unconformity

CROSS SECTION OF FRENCHMAN

MOUNTAIN.

Vishnu granites and the overlying Tapeats Sandstone

http://www.wheaton.edu/ACG/trip/stop2a.html

•Clarence Dutton in his 1882 book Tertiary History of the Grand Cañon District.

•People assume that the Great Unconformity was so named due to the large

interval of time it represents in the Earth’s history.

•The truth is Dutton had no idea of the age of the rocks on either side of the

Unconformity.

•Basically, he knew the importance of this find and that something was

missing but had no idea of the extent of time that had passed between the

Cambrian age Tapeats sandstone and the granite and schist that lie beneath it.

WHERE DID THE NAME “GREAT

UNCONFORMITY” COME FROM?

http://geoscience.unlv.edu/pub/rowland/Virtual/geology.html#name

Great Unconformity Geologic Interpretive Site and Trail

http://geoscience.unlv.edu/pub/rowland/Virtual/virtualfm.html

http://www.flickr.com/photos/kitkattt9/2061774736/

http://www.flickr.com/photos/kitkattt9/2061775502/

West face

Igneous laccolith, Lava Butte south of Frenchman Mtn, Alluvial fans and Lake Mead.

http://www.idahosummits.com/frenchman/images/lake%20mead.jpg

Eastern Face of Frenchman Mtn. with Las Vegas in the foreground.

Peak of Frenchman mtn.